Japanese Patent Unexamined Publication No. 2003-185719 discloses a control device of a secondary battery as set forth below. That is, the control device of the secondary battery defines a predetermined battery model, and a current measured value and a terminal voltage measured value of the secondary battery are converted into a state quantity by using a state variable filter which is based on a battery model. By using the state quantity, the control device of the secondary battery estimates the secondary battery's terminal voltage which is based on the battery model. Then, the control device of the secondary battery identifies a parameter of the secondary battery such that a difference between the voltage measured value and the terminal voltage estimated based on the battery model is converged to zero.